Reading Cringely's column, I came across gumstix, a line of very small computers.
Now, the last time I messed with reengineering a REAL RPN calculator, I cross-wired a HP-41C keyboard and my TI-83+SE . I figured the 41's case and quality keypad might serve as a good starting point -- I just needed programmable guts and a display. Alas, the TI's innards, while flash-programmable and well supported, won't quite fit within the 41's case (PCB width being the the kicker).
I still think that basic approach deserves hobbyist attention, and I'll be taking another look -- this time with gumstix. (In my "spare time", of course.)